This invention relates to power generating systems and more particularly to such systems which include permanent magnet alternators.
In many applications, there is a need for an efficient power supply with a controllable output frequency. One such application is an airborne power generating system where the prime source of electrical power is a rotating generator that receives its mechanical power input from the engine of the aircraft. Since the engine speed varies, usually over a 2 to 1 range, it is not possible for the generator to produce constant frequency output if coupled directly to the engine. One method of forming a constant frequency output has been to insert a hydraulic constant speed coupling device between the engine and the generator, thereby enabling the generator to operate at a constant speed and hence produce electrical power at a constant frequency. Such a system has several disadvantages, not the least of which is relatively frequent and costly maintenance. These disadvantages can be overcome by an alternative approach in which the generator is directly coupled to the engine, allowing it to produce variable frequency output power as dictated by the engine speed. This variable frequency power is then converted into accurately regulated constant frequency output power by means of a static frequency converter.
One type of system which allows the generator to be coupled directly to the aircraft engine is a DC-link variable speed constant frequency (VSCF) system. In a DC-link converter arrangement, the alternating voltage of the generator is converted into a direct voltage by a rectifier circuit. Then the direct voltage is converted back to alternating voltage, at the desired frequency, by a static power inverter. DC-link VSCF systems usually include synchronous generators which are high speed, spray and conduction oil cooled, three phase output machines. A three phase full-wave rectifier converts the varying frequency (typically 1100 to 2500 Hz) output AC voltage of the generator to a DC voltage. This DC-link voltage is used by the power electronics module to generate a three phase, constant frequency system output voltage. The amplitude of the DC-link voltage is controlled by the magnitude of the field excitation current in the generator. During system operation, the exciter field current is close-loop controlled to maintain the system output voltage within specified limits.